Fire-retardant compositions are well known for decreasing the flammability or combustibility of materials, in particular wood and wood products, and for increasing the resistance of these materials to heat and flame damage. Wood and wood products have numerous desirable qualities as construction materials, including relatively low cost, structural strength, paint-ability and stain-ability, insulating properties, wide availability, renewability of the resource, and pleasing aesthetically characteristics. As a result, wood and wood products are used extensively as building materials for residential and commercial applications by the construction industry. Flammability, however, is the most notable disadvantage of using wood and wood products as construction materials. The susceptibility of wood to fire-related damage leads to millions of dollars per year in property damage, and also produces significant human injury and loss of life.
A number of building codes, for example, the International Residential Code (IRC), the Life Safety Code (NFPA 101), and the Building Construction and Safety Code (NFPA 5000), recognize that wood impregnated with fire retardant compositions that meet certain performance criteria may be used in place of noncombustible materials for exterior walls of Type I, II, III and buildings and in roof structures of type II and low-rise buildings of Type I construction (NFPA 5000). Most of these building codes require fire-retardant treated wood (FRTW) to perform to certain levels in accordance with tests set out in ASTM E-84 (“Standard Test Method of Surface Burning Characteristics of Building Materials”), NFPA 255 (“Standard Method of Test of Surface Burning Characteristics of Building Materials”) or UL 723 (“Standard for Test for Surface Burning Characteristics of Building Materials”)(each incorporated herein by reference in their entireties). Although the standard flame-spread test in ASTM E-84, for example, is based on a 10-minute exposure in a fire test tunnel furnace, under controlled conditions of draft and temperature, as specified in ASTM E-84, the test period for FRTW is extended to 30 minutes to confirm that the wood does not demonstrate significant progressive combustion. According to these tests, wood designated FRTW must demonstrate surface burning characteristics in a 30-minute extended burn test that the “flame spread index shall be 25 or less and there shall be no evidence of significant progressive combustion when the test is continued for an additional 20-minute period. Additionally, the flame front shall not progress more than 10½ feet (3200 mm) beyond the centerline of the burners at any time during the test. The smoke-developed index shall be 450 or less.”
Generally, commercial fire-retardant formulations for pressure impregnating wood products contain: (1) various phosphate compounds, including mono-ammonium phosphate, diammonium phosphate, ammonium polyphosphate and metal salts of phosphoric acid; (2) sulfate compounds, such as ammonium sulfate, copper sulfate, and zinc sulfate; (3) halogenated compounds, such as zinc chloride and ammonium bromide; (4) nitrogen compounds, such as dicyandiamide and urea; or (5) boron compounds, such as boric acid, sodium borates or other metal borates.
Phosphate-based fire retardant compositions have long been used to confer fire retardant properties onto wood impregnated with such composition and are very effective fire-retardant chemicals. Phosphate compounds raise concerns with respect to their effect on the structural integrity of wood and wood products, especially at higher loading in wood products. Phosphate compounds hydrolyze into phosphoric acid when exposed to prolonged heat and moisture and may react with the wood and degrade the treated wood structure through an acid degradation reaction which reduces the mechanical strength of the treated wood over time. The generation of phosphoric acid in wood degradation is enhanced in environments of elevated temperature and moisture such as in roof and attic areas. Higher loading of phosphate-based fire retardants also increases the hygroscopicity of the treated wood. Increased hygroscopicity and increased generation of phosphoric acid can impact the structural integrity of the treated wood. Many building codes also require other tests to assess the Flexural Strength and Stiffness Properties (ASTM D5516) of wood, its hygroscopicity (ASTM D3201) and its corrosiveness (American Wood Protection Association (AWPA) E-12 procedure).
For example, U.S. Pat. No. 3,832,316 to Juneja discloses a fire retardant for wood consisting of melamine, phosphoric acid, dicyandiamide and formaldehyde. The same inventor, Juneja, also discloses a fire-retardant composition for wood in the Canadian Patent No. 917,334 comprising urea, phosphoric acid, dicyandiamide and formaldehyde.
Several other patents, including U.S. Pat. Nos. 4,010,296; 3,137,607; and 2,935,471, describe fire-retardant compositions comprising dicyandiamide and phosphoric acid in free form or a phosphate. U.S. Pat. No. 2,917,408 to Goldstein et al., describes a fire retardant compositions for use on wood having a phosphorus-amine complex, which is a combination of phosphoric acid and dicyandiamide. Similarly, U.S. Pat. No. 3,159,503 to Goldstein et al. uses a combination of formaldehyde, phosphoric acid and dicyandiamide to impart fire-retardant properties to wood. In a slightly different approach, U.S. Pat. No. 6,652,633 discloses a fire-retardant composition based on guanylurea phosphate and boric acid. As can be deduced from these examples, a vast majority of fire-retardant compositions contain phosphoric acid or reaction by-products of phosphoric acid. U.S. Pat. No. 4,725,382 discloses a water soluble fire retardant composition containing phosphate compounds and boron compound for pressure impregnation. U.S. Pat. No. 5,151,225 discloses a fire retardant composition comprising oxyacid of phosphorus, a borate compound and an amide compound with a pH range of 4.75 and 5.25. U.S. Pat. No. 4,461,720 discloses a fire retardant composition containing a solution of methylated guanyl urea and melamine with molar ratio of guanyl urea to melamine in the range of 5:1 to 10:1. Oberley U.S. Pat. No. 4,373,101 discloses a fire retardant composition comprising mixture of boric acid and a partial reacting product of dicyandiamide and phosphoric acid. Several additional examples of such phosphoric acid- or phosphate-containing fire retardants include U.S. Pat. Nos. 4,373,010; 4,514,326; and 4,725,382. Alternatively, U.S. Pat. Nos. 6,517,748 and 6,306,317 disclose phosphoric acid-free/phosphate-free fire-retardant formulations containing nitrogen compounds and boron compounds.
Nitrogen and boron compounds also raise concerns when used in fire-retardant formulations for treating wood. Nitrogen compounds, such as urea and dicyandiamide, have undesirable hygroscopic properties. In high concentration or high chemical loading in wood products, these chemicals can draw moisture from the air making the treated wood very hygroscopic. The undesirable hygroscopic property can adversely causes chemical blooming out from the treated wood, more corrosion to metal fasteners, and thermal degradation of wood cellulose fiber when used along with phosphate based compounds.
The industry uses coatings for treating wood products to provide them with a fire rating.
Commercial formulations for coating wood products for the purposes of fire ratings are well known in the art. Generally, such coatings comprise one or more polymer binders, a mineral acid catalyst, a carbon source, and a source of non-flammable gas (i.e. a blowing agent or a foaming agent).
As described above, the industry uses either fire-retardant impregnates to confer fire retardance to wood, or uses coatings to provide a fire rating. In addition to the disadvantages discussed above with those methods, certain wood products do not pass the ASTM E-84 30-minute burn test.
Despite many efforts to address these deficiencies in fire-retardant formulations, there remains an unmet need to develop a fire-retardant technology for wood products with sufficient fire-retardant properties to pass industry and code-specified tests for fire retardance and suitable for commercial use. For example, the optimal fire retardant should be less hygroscopic and less corrosive to metal fasteners, has long-term thermal stability, and imparts excellent fire-retardant characteristics to wood based products. This need is addressed by the invention disclosed herein.
The instant invention is also advantageous because it minimizes or even eliminates the use of phosphate compounds. This is advantageous because it avoids the drawbacks of the prior art where wood treated with phosphate-based fire retardant compositions degraded when exposed to prolonged heat and moisture. Phosphorus and, in particular, phosphate-containing compounds can lead to the formation of phosphoric acid which, over time, will degrade wood.